Born in Sligo, Christopher O'Donnell grew up primarily as a football player, representing his local side Benbulben FC and the Sligo/Leitrim Youth Schoolboys League.

Operating mainly as a winger, O'Donnell was selected to represent the Sligo/Leitrim Youth Schoolboys League at the prestigious SFAI Kennedy Cup tournament in Limerick in June 2012.

He attempted to juggle both sports for a couple of years before eventually deciding to focus his efforts full-time on track and field at the age of 16 in 2015.

In the relay he was part of the silver medal-winning team which finished with a time one-hundredth of a second outside of the Irish U18 record.

In 2015, O'Donnell had his first taste of the 400 metres, and despite finishing runner-up in the national U18 indoors, his time of 50.25 was under the previous Championship record held by Brian Gregan.

That summer seen a big breakthrough over 400m where he won his race at the IFAM International in Oordegem in 46.92, which was an "A" standard for the European U20 Championships in Grosseto.

In July 2019, following disappointment at the European U23 Championships and failing to improve on his personal best times under Dakin, O'Donnell returned to Sligo to be coached by Gaynor again on an interim basis until the end of the season.

A limited season resulted with no International Championships due to the COVID-19 pandemic, but he still managed to improve his personal bests in the 400m and 200m, while also winning the national 400m title (1st U23) for the third year in a row, and bronze in the 200m (2nd U23).

He parted company with Baker during the season and teamed up with Swedish sprints coach Benke Blomkvist in Loughborough following the Olympic Games.